The present disclosure relates generally to semiconductor manufacturing operations, and more particularly to an improved method and system for making photo masks with oblique features to be used in a photolithography process.
Modern microelectronic devices are commonly produced using a photolithographic process to imprint circuit designs on a wafer substrate. In this process, a semiconductor wafer is first coated with a layer of photoresist. This photoresist layer is then exposed to illuminating light with a photomask image (for simplicity, the terms photomask, mask, and reticle will be used interchangeably and with equivalency) and subsequently developed. After the development, the patterned photoresist produces an image of the mask on the wafer. Thereafter, the uppermost layer of the wafer is etched, implanted or otherwise processed, and the remaining photoresist is stripped. For multilayer wafers, the above procedure is then repeated to produce subsequent patterned layers. As such, the quality of the masks are important because it is directly related to the quality of the circuits because the accurate conversion from the circuit design to the semiconductor wafer only happens when there is a quality set of masks.
In the past, for various reasons including consideration for quality control, circuits have been designed to have only horizontal and vertical lines so that they are easy to be produced on wafers. Consequently, there are largely horizontal and vertical lines on the masks. As the circuits are shrunk smaller and smaller, the space on each die of the wafer becomes precious. It is now sometimes desirable to have oblique lines to make connections than using vertical and horizontal lines for the same purpose. Although it is not much trouble to use oblique orientation to layout semiconductor integrated circuits on the design in order to be efficient with the cell size, it is, however, difficult to produce these oblique lines on masks as all the existing mask-making machines are designed for crafting vertical and horizontal lines with critical precision on the mask.
FIG. 1 illustrates a top view of a typical mask writer system 100. Since only vertical and horizontal lines are expected to be produced, a mask stage 102, a mask holder 104 on the mask stage, and a blank mask or mask or reticle substrate 106, which is held by the mask holder 104, are all designed to be accurately aligned in both the horizontal reference direction (or X direction) and the vertical reference direction (or Y direction). The alignment of these components can be accomplished by using two mirrors 108 and 110 reflecting lights to an X interferometer 112 and Y interferometer 114. In this conventional configuration, the mask holder 104 is solidly fixed on the mask stage 102 so that when the mask stage is aligned, everything else is also aligned. Once aligned, the mask writer 100 can craft mask pattern 116 on the blank mask as desired. The mask patterns such as the pattern 116 in FIG. 1 may contain oblique features such as oblique lines with respect to the X-Y alignment configuration.
FIGS. 2A-2C illustrate how conventional mask writers handle the oblique lines. Most mask writers handle such oblique lines by combining small oblique patterns together instead of using a large continuous area. For example, as shown in FIG. 2A, an oblique line 200 is formed by numerous triangles 202. As shown in FIGS. 2B and 2C, the connection between an oblique line 204 and a horizontal line 206 (or vertical line) can be done by overlapping these two entities or by adding a third oblique entity such as a triangle 208 in between. Since crafting any oblique entity is time consuming, the throughput of the mask writer and the pattern fidelity of the produced pattern are negatively impacted.
Further, for semiconductor manufacturing tools requiring dimension measurement, the feature of interest is placed vertically. For example, these tools include metrology tools like CDSEM, optical measurement microscopes, mask inspection tools, mask repair tools, and mask verification tools. The dimensions within a range of cross sections are evaluated by either scanning an e-beam or an optical spot across.
Even after a mask is produced, when verifying the quality of the mask, a typical mask verification tool mimics imaging parameters of the mask pattern replication tools. For example, an Aerial Image Simulation (AIMS) tool uses a small field lens having the equivalent numerical aperture together with the same wavelength and illumination condition of the actual wafer-imaging tool that will replicate the mask pattern on wafers. The aerial image from the AIMS tool is examined for critical dimension (CD) control, bridging or breaking. It is understood that if the feature of interest is placed obliquely instead of vertically or horizontally, the measurement is less accurate and more time consuming in view of the fact that the measuring and scanning grids of these tools are all vertically and horizontally oriented. Similarly, the observation and pattern-modification functions in mask repair tools also prefer horizontal and vertical patterns for assuring better accuracy.
Because of the tool limitations, most layout patterns are oriented in horizontal or vertical reference directions. At the very least, all critical patterns are oriented as such. Only patterns of relaxed dimension are allowed to be obliquely oriented. It has been a difficult task to craft oblique critical patterns on a mask.
What is needed is an improved method and system for producing oblique patterns while still maintaining the pattern dimension accuracy so that the produced mask can be used in semiconductor manufacturing operations.